For the purpose of operating modern internal combustion engines and ensuring compliance with strict emission limiting values, an engine controller determines, by means of what is referred to as a cylinder charge model, the air mass which is enclosed in a cylinder per working cycle. The appropriate fuel quantity setpoint value (MFF_SP) is injected via an injection valve, also referred to in this document as an injector, in accordance with the modeled air mass and the desired ratio between the air quantity and the fuel quantity (lambda). In this way, the fuel quantity to be injected can be dimensioned in such a way that a lambda value which is optimum for the exhaust gas post-treatment in the catalytic converter is present. For direct-injection spark ignition engines with internal mixture formation, the fuel is injected directly into the combustion chamber at a pressure in the range from 40 to 200 bar.
The main requirement made of the injection valve is, in addition to the seal with respect to an uncontrolled outflow of fuel and the conditioning of the jet of the fuel to be injected, precisely timed metering of the pilot-controlled injection quantity.
In particular in the case of supercharged direct-injection spark ignition engines, a very large quantity spread of the required fuel quantity is necessary. It is therefore necessary, for example for the supercharged operating mode at the full load of the engine, to meter a maximum fuel quantity MFF_max per working cycle, while in the operating mode near to idling a minimum fuel quantity MFF_min has to be metered. The two characteristic variables MFF_max and MFF_min define here the limits of the linear working range of the injection valve. This means that there is a linear relationship between the injection time (electrical actuation period (Ti)) and the injected fuel quantity per working cycle (MFF) for these injection quantities.
For direct injection valves with a coil drive, the quantity spread, which is defined as the quotient between the maximum fuel quantity MFF_max and the minimum fuel quantity MFF_min when the fuel pressure is constant, is approximately 15. For future engines with the emphasis on CO2 reduction, the cubic capacity of the engines is reduced and the rated power of the engine is maintained or even increased by means of corresponding engine charging mechanisms. As a result, the demands which are made of the maximum fuel quantity MFF_max correspond at least to the demands made of an induction engine with a relatively large cubic capacity. However, the minimum fuel quantity MFF_min is determined, and therefore reduced, by means of operation near to idling and the minimum mass of air under overrun conditions of the engine with a reduced cubic capacity. In addition, direct injection permits the entire fuel mass to be distributed over a plurality of pulses, which, for example in a catalytic converter heating mode, permits more stringent emission limiting values to be complied with by what is referred to as mixture stratification and a later ignition time. For the above-mentioned reasons, for future engines increased demands will be made both of the quantity spread and also of the minimum fuel quantity MFF_min.
In known injection systems, a significant deviation of the injection quantity from the nominal injection quantity occurs in the case of injection quantities which are smaller than MFF_min. This systematically occurring deviation is due essentially to fabrication tolerances at the injector and to tolerances of the output stage, which actuates the injector, in the engine controller, and therefore to deviations from the nominal actuation current profile.
The electrical actuation of a direct injection valve typically takes place by means of a current-controlled full-bridge output stage. Only a limited level of accuracy of the current profile which is applied to the injector can be achieved under the peripheral conditions of a vehicle application. The resulting variation in the actuation current, and the tolerances at the injector, have significant effects on the achievable accuracy of the injection quantity, in particular in the region of MFF_min and below.
The characteristic curve of an injection valve defines the relationship between the injected fuel quantity MFF and the time period Ti of the electrical actuation (MFF=f(Ti)). The inversion of this relationship Ti=g(MFF_SP) is used in the engine controller to convert the setpoint fuel quantity (MFF_SP) into the necessary injection time. The additional influencing variables which are included in this calculation, such as the fuel pressure, the internal pressure of a cylinder during the injection process, as well as possible variations in the supply voltage, are omitted here for the sake of simplification.
FIG. 7a shows the characteristic curve of a direct injection valve. Here, the injected fuel quantity MFF is plotted as a function of the time period Ti of the electrical actuation.
As is apparent from FIG. 7a, for time periods Ti which are greater than Ti_min there is a working range which is linear to a very good approximation. This means that the injected fuel quantity MFF is directly proportional to the time period Ti of the electrical actuation. For time periods Ti which are shorter than Ti_min there is a highly nonlinear behavior. In the illustrated example, Ti_min is approximately 0.5 ms.
The gradient of the characteristic curve in the linear working range corresponds to the static flow through the injection valve, i.e. the fuel flow rate, which is continuously attained during the entire valve stroke. The cause of the nonlinear behavior for time periods Ti which are shorter than approximately 0.5 ms or for fuel quantities MFF<MFF_min is, in particular, in the inertia of an injector spring-mass system and the chronological behavior during the building up or reduction of the magnetic field by a coil, which magnetic field activates the valve needle of the injection valve. As a result of these dynamic effects, the entire valve stroke is no longer achieved in what is referred to as the ballistic range. This means that the valve is closed again before the structurally predefined final position, which defines the maximum valve stroke, has been reached.
In order to ensure a defined and reproducible injection quantity, direct injection valves are usually operated in a linear working range. At present, operation in the nonlinear range is not possible since, owing to the above-mentioned tolerances in the current profile and mechanical tolerances of injection valves (for example the prestressing force of the closing spring, stroke of the valve needle, internal friction in the armature/needle system), a significant systematic error occurs in the injection quantity. For reliable operation of an injection valve, this results in a minimum fuel quantity MFF_min per injection pulse, which minimum fuel quantity MFF_min has to at least be provided in order to be able to implement the desired injection quantity precisely in terms of quantity. In the example illustrated in FIG. 7a, this minimum fuel quantity MFF_min is somewhat smaller than 5 mg.
The electrical actuation of a direct injection valve usually occurs by means of current-regulated full-bridge output stages of the engine controller. A full-bridge output stage makes it possible to apply an on-board power system voltage of the motor vehicle to the injection valve, and alternatively to apply a boosting voltage thereto. The boosting voltage is frequently also referred to as boost voltage (U_boost) and can be, for example, approximately 60 V.
FIG. 7b shows a typical current actuation profile I (thick continuous line) for a direct injection valve with a coil drive. FIG. 7b also shows the corresponding voltage U (thin continuous line) which is applied to the direct injection valve. The actuation is divided into the following phases:
A) Pre-charge phase: During this phase with a duration t_pch, the battery voltage U_bat, which corresponds to the on-board power system voltage of the motor vehicle, is applied to the coil drive of the injection valve by means of the bridge circuit of the output stage. When a current setpoint value I_pch is reached, the battery voltage U_bat is switched off by a two-point regulator, and U_bat is switched on again after a further current threshold is undershot.
B) Boost phase: The pre-charge phase is followed by the boost phase. For this purpose, the boosting voltage U_boost is applied to the coil drive by the output stage until a maximum current I_peak is reached. As a result of the rapid build up in current, the injection valve opens in an accelerated fashion. After I_peak has been reached, a freewheeling phase follows until the expiry of t_1, during which freewheeling phase the battery voltage U_bat is in turn applied to the coil drive. The time period Ti of the electrical actuation is measured starting from the beginning of the boost phase. This means that the transition to the freewheeling phase is triggered by the predefined maximum current I_peak being reached. The duration t_1 of the boost phase is permanently predefined as a function of the fuel pressure.
C) Commutation phase: After the expiry of t_1, an off-commutation phase follows. Here, a self-induction voltage, which is substantially limited to the boost voltage U_boost, is produced as a result of the switching off of the voltage. The limitation of the voltage during the self-induction is composed of the sum of U_boost, of the forward voltages of a recuperation diode and of what is referred to as a freewheeling diode. The sum of these voltages is referred to below as recuperation voltage. The recuperation voltage in the commutation phase is represented negatively on the basis of a differential voltage measurement on which FIG. 7b is based.
As a result of the recuperation voltage, a current flow occurs through the coil and reduces the magnetic field. The commutation phase is timed and depends on the battery voltage U_bat and on the duration t_1 of the boost phase. The commutation phase ends after the expiry of a further time period t_2.
D) Holding phase: The off-commutation phase is followed by what is referred to as the holding phase. Here, in turn, the holding current setpoint value I_hold is adjusted by means of the battery voltage U_bat by means of a two-point controller.
E) Switch-off phase: As a result of the voltage being switched off, a self-induction voltage occurs which, as explained above, is limited to the recuperation voltage. This results in a current flow through the coil, which current flow then reduces the magnetic field. After the recuperation voltage which is illustrated negatively here is exceeded, current no longer flows. This state is also referred to as open coil. Owing to the ohmic resistances of the magnetic material, the eddy currents which are induced when the field of the coil is reduced decay. The reduction in the eddy currents leads in turn to a change in the field in the magnetic coil and therefore to a voltage induction. This induction effect leads to a situation in which the voltage value at the injector rises to zero starting from the level of the recuperation voltage according to the profile of an exponential function. After the reduction of the magnetic force, the injector closes by means of the spring force and by means of the hydraulic force which is caused by the fuel pressure.
The described actuation of an injection valve has the disadvantage that the precise time of closing of the injection valve or of the injector in the open coil phase cannot be determined. Since a variation in the injection quantity correlates to the resulting variation in the closing time, the absence of this information, in particular at very small injection quantities which are smaller than MFF_min, results in a considerable degree of uncertainty with respect to the fuel quantity which is actually introduced into the combustion chamber of a motor vehicle engine.